In recent years, active flow control has been used to increase the aerodynamic efficiency of machines having air flow over a surface, in particular vehicles such as airplanes. Adverse fluid flows generated over aerodynamic surfaces can buffet and fatigue downstream structures exposed to the flows, and the flows can affect efficiency by increasing drag or resistance over the surface. In one version of active flow control, jets of air are blown into the path of the adverse fluid flows to mix with the flows and cause the air to flow more smoothly over the aerodynamic surfaces and reduce the drag and resistance over the surfaces. In many cases, such active flow control can be implemented in existing vehicle designs without needing significant changes thereby directly reducing the operating cost of the vehicle or other machine.
One device for creating jets of air in active flow control is a synthetic jet that forms a jet flow by moving air back and forth through a small opening of the device. Synthetic jets typically have a housing in the shape of a hollow box or cylinder with a resonant chamber therein and an orifice or nozzle opening through one of the side or end walls. At least one wall of the synthetic jet is formed from a flexible membrane that can deflect inwardly and outwardly to alternately decrease and increase the volume in the resonant chamber and expel and draw in air through the opening. Deflection of the membrane may be caused by a piezoelectric actuator that responds to an applied electric field. The piezoelectric actuator may include a piezoceramic plate having a surface facing and rigidly attached to a corresponding surface of the membrane. The actuator may have a single piezoceramic plate attached to a surface of the membrane, or two piezoceramic plates with each plate being attached in a similar manner to one of the opposing surfaces of the membrane.
For each piezoceramic plate, electrodes are attached to the opposing planar surfaces for application of the electric field across the thickness of the plate. Due to the converse piezoelectric effect, the applied electric field causes stresses and mechanical deformation through the thickness of the plate, and corresponding stresses and mechanical deformation occur in the plane of the plate due to the Poisson effect. In-plane deformation of the plate creates bending moments on the surface of the membrane and deflection of the membrane relative to the resonant chamber. Alternating the polarity of the electric fields across the plates causes the plates to alternately compress and elongate. Alternating the electric field at a high frequency causes rapid vibration of the membrane and creation of high velocity flow through the opening of the synthetic jet.
In piezoelectric actuators as described, the electric field is applied through the thickness of the piezoceramic plates, but the actuators rely on the in-plane properties and mechanical deformation of the plate to apply the moment that bends the membrane. However, due to the Poisson ratios for the commonly used isotropic piezoceramics, the in-plane piezoelectric d coupling coefficients, which are ratios of the mechanical strain developed versus the applied electrical field, are between a factor of two to three times less than the transverse piezoelectric d coupling coefficient. As a result, the in-plane strains are one half to one third less than the transverse strains for an applied electrical field, thereby inherently limiting the response of the actuators to the electric energy applied across the thickness of the piezoceramic plates. In view of this, a need exist for improved piezoelectric actuators that are more efficient in utilizing the electric energy available for operation.